Encapsulation of He and Ne in Carbon Molecular Sieves

Abstract

The adsorption-desorption pattern of He and Ne from amorphous carbon molecular sieve fibers (CMSF) was found to be governed by an encapsulation mechanism. He and Ne undergo reversible and efficient entrapment by the micropores of CMSF. Selective adjustment of pore openings to meet the critical dimensions of He and Ne which allow their admission is achieved via two principal steps: (i) the well-known pore widening by means of an irreversible removal of surface oxide groups upon evacuation at elevated temperatures and (ii) regulation of the effective pore opening via reversible thermal dilation/contraction. The occurrence of a markedly high activation energy, which previously could not be solely attributed to pure adsorption, is now understood as that needed for overcoming the energetic barrier imposed by the thermally constricted pores. The present results imply that the encapsulation phenomenon can considerably affect dead volume measurements in porous materials.

Mesoporous Silicon in Drug Delivery Applications

“…In the case of partially blocked pores, pycnometry might give more realistic results than the sorption method. However, there are some possible sources of error, for example trapping of He has been reported, especially if the sample contains micropores 147–149…”

A scanning tunnelling microscopy insight into the preparation of carbon molecular sieves by chemical vapour deposition

“…Carbon molecular sieves (CMSs) are an important class of microporous carbon materials with special textural characteristics that make them extremely attractive not only for applications, e.g., in gas purification and separation devices, [1][2][3][4] but also from a fundamental science perspective. [5][6][7][8][9] For a CMS to be effective for the separation of a certain gas mixture, two main requirements have to be met: molecular selectivity and sufficiently high adsorption capacity for the species to be retained. The former requisite is achieved using carbons with narrow pore size distributions (PSDs) and pore mouths of molecular dimensions (typically a few A ˚), whereas to fulfil the latter, the material must possess a reasonably large micropore volume.…”

A Review of Conventional and Emerging Process Technologies for the Recovery of Helium from Natural Gas

“…The entrapment of He in carbon molecular sieve fibres and Type A zeolites at low pressures and temperatures from 77 K to room temperature has been reported by a research group (Finkelstein et al 2003;Saig et al , 2005. Although the absolute volume of He adsorbed at low pressures was small, the materials did adsorb some He at low pressures, and reasonable He uptakes were observed at higher pressures.…”